Commercially available LEDs typically emit a single color (peak wavelength). Some experimental LED chips have been produced that can produce multiple peak wavelengths. However, such LEDs are fabricated using special techniques that add significant expense, making them not yet commercially viable.
The most common white-light LED uses a 450 nm blue LED that excites a relatively broad emission YAG:Ce phosphor deposited over the LED. The peak emission wavelength of YAG is about 535-565 nm (yellow-green). The blue light leaks through the YAG layer, and the combination of the blue and yellow light (phosphor converted blue light) creates white light. Such a di-chromatic light source produces relatively poor color rendering, since the highest color rendering is produced with a broad-spectrum source, such as the sun or a radiating black body.
The color rendering index (CRI) is a quantitative measure of the ability of a light source to faithfully reproduce the colors of certain sample patches in comparison with an ideal or natural light source, such as the sun, at the same color temperature. The sun has a CRI of 100.
It is known to use a blue LED to excite a combination of red, yellow, and green phosphors to produce a broader spectrum having a high CRI. However, combining and depositing phosphors to produce a consistent emitted color is difficult. Phosphors are also relatively expensive and reduce the efficiency of the LED. Additionally, the environmental impact of mining phosphors is significant and growing due to the increased use of phosphors.
Another problem with phosphors is that the color cannot be controlled once the phosphor is deposited over the LED.
It is known to combine red, green, and blue LEDs in a module to produce white light, but each LED needs its own current source, and the different LEDs react differently to heat. Accordingly, such LED modules have better CRI but lack color stability and are expensive.
What is needed is a technique for fabricating a white light source that improves CRI, luminous efficiency, and color control without the drawbacks discussed above.